The invention relates to a method and to a circuit arrangement for implementing a synchronization method in a receiver of digital data transmissions including a timing error detector to which are fed in in-phase component and a quadrature component of a demodulated received signal produced from products of the received signal and an output signal of a carrier oscillator and lowpass filtering of the products to suppress harmonics. An output signal of the timing error detector serves as a control signal u.sub.TI for a clock pulse generator which produces clock pulses for sampling the received signal. The control signal u.sub.TI is produced by means of two bandpass filters in the timing error detector which convert the demodulated received signal into two respective complex signals which are in turn linked together by means of a linkage circuit in the timing error detector to form the control signal u.sub.TI. The control signal u.sub.TI is used to actuate a timing error correction circuit of the clock pulse generator to synchronize the clock pulses with the received signal.
Such methods are known, for example from the article by Gardner, entitled "A BPSK/QPSK Timing-Error Detector for Sampled Receivers," in IEEE Com.-34, No. 5, May, 1986, pages 423-429.
In digital data transmission it is important for the receiver to derive the exact sampling moment from the received data. For this purpose, a timing error detector generates by way of non-linear operations an output signal u.sub.TI whose fundamental mode corresponds to the clock pulse frequency and whose zero passages permit derivation of the sampling moment.
FIG. 1 shows a clock pulse control loop as disclosed in the above Gardner article and as it is employed, for example, for QPSK (quadrature phase shift keyed) data transmission. The incoming received signal s(t) is demodulated in that the carrier signal is fed to it multiplicatively once directly and once with a phase shift of -.pi./2. By means of lowpass filters TP, the products of the double frequency are suppressed. The normal component, i.e. the real component x(t), and the quadrature component, i.e. the imaginary component y(t), of the demodulated input signal are obtained from the outputs of the lowpass filters and fed to a subsequent timing error detector TD. Its output signal u.sub.TI is fed via a loop filter (SF) to a timing error correction device TPK which actuates a sampling device A for sampling, for example, the non-demodulated or the demodulated input signal. Such sampling is necessary if the signals are to be processed in the receiver in a time discrete manner.
In FIG. 1, for example, sampling occurs before demodulation. However, such sampling could also be done at any other desired location, for example after demodulation or after lowpass filtering. It is here assumed that the output signal of the timing error detector is present as a sampled signal u.sub.TI (kT), where T is the symbol interval or the step length which, according to the relationship of f.sub.Nyq =1/2T is a function of the Nyquist frequency f.sub.Nyq. The sampling phase of the sampler A present in the receiver is changed by way of loop filter SF and timing error correction circuit TPK until the sampled values at the output of the timing error detector average zero: u.sub.TI (k.multidot.T)=0, where k is an integer which numbers the clock pulses.
FIG. 2 shows the typical characteristic of a timing error detector. The sampling phase .epsilon. , i.e. the deviation .DELTA.t/T of the actual sampling moments from the correct sampling moments (in the case where transmitter and receiver clock pulse are coherent, .epsilon.=0) are plotted in the direction of the abscissa and the averages of u.sub.TI are plotted in the direction of the ordinate. For .epsilon.=0 and .epsilon.=.+-.0.5, u.sub.TI =0. The center .epsilon.=0 of this sinusoidal curve is stable while at .epsilon.=.+-.0.5, synchronization may be labile, i.e. it is possible that the synchronization is caught at the labile point for a longer or shorter period of time, thus noticeably lengthening the synchronization time.